Biological prostheses are medical devices that utilize animal tissues. Examples of suitable animal tissues include bovine, porcine, ovine and equine. Depending on the various medical uses, the biological tissue include cardiac valves, pericardium, tendons, ligaments, dura mater, skin, veins, etc.
The animal tissues used in biological prostheses are formed primarily of collagen, a protein with a structural unit represented by three polypeptide chains that associate to form a triple helix. Collagen molecules assemble to form microfibrils that in turn assemble to form fibrils that, arranged in corrugated or parallel bundles, give rise to true collagen fibers. Such tissues have good resistance to traction and are flexible but substantially inextensible.
Animal tissues used in biological prostheses are first subjected to numerous washings to eliminate traces of blood and a careful removal of adipose and ligamentous parts. However, cells or cellular residues from the animal donor can remain trapped in the structure of the tissue itself. As a result it is possible that the immune system of the host gives rise to a rejection phenomenon that can lead to the destruction of the tissue constituting the biological prosthesis.
An additional problem is degradation of the collagenous biological tissue once implanted in the host organism. For this reason, the biological tissues are subjected to a fixation treatment with the aim of protecting the tissue from such degradation phenomena and contributing to preventing the above-mentioned rejection phenomenon.
Among the substances used for the fixation of biological tissues, the most common is glutaraldehyde. This bifunctional molecule, carrying two aldehyde groups, is capable of stably binding together free amino groups of the amino acids that constitute the polypeptide chains both within one collagen molecule and between adjacent collagen molecules. In this way glutaraldehyde forms intra-chain and inter-chain bridge structures, causing the cross linking of biological tissue. Such cross linking protects the tissue from degradation by the host and confers favorable mechanical properties such as for example a better resistance to traction with respect to untreated tissue.
Glutaraldehyde is a highly bactericidal and virucidal substance; therefore, in addition to cross linking the tissue, the fixation step also provides at least a partial sterilization.
In addition, glutaraldehyde is capable of binding to the free amino residues of the membrane proteins of the cellular components still present, masking their antigenic potential and impeding immune activation phenomena and rejection by the host.
In spite of widespread use, glutaraldehyde has a disadvantage of being one of the factors that favors pathological calcification of implanted tissues. The calcium, present in the bodily fluids of the host organism, accumulates in proteinaceous tissue giving rise, for example in the case of biological cardiac valves, to a process that may represent one of the principle causes of valve failure. The calcium deposits can reduce the flexibility of the portion of biological tissue constituting the valve (or the so-called valve leaflets or cusps) and lead to laceration of the tissue itself, causing a partial or total loss of valve function.
The mechanism responsible for calcification is not yet completely known and is attributed to numerous factors; however, it is known that after glutaraldehyde fixation, aldehyde groups remaining free on the tissue can create binding sites for calcium.
In addition, the toxicity of such aldehyde residues can cause local inflammatory phenomena that lead to the necrosis of host cells. Destruction of the dead cells gives rise to cellular debris that, in turn, can constitute binding cites for calcium. Several types of molecules capable of neutralizing the aldehyde residues remaining free after the fixation process have been used to limit the process of tissue calcification. For example, the use of amino acids has been shown to have an anti calcification effect; in particular, U.S. Pat. No. 5,873,812 describes the use of amino carboxylic acids, such as for example homocysteic acid, in the preparation of aldehyde-fixed biological tissues. However, this method only partially neutralizes free aldehyde groups and thus does not resolve the problem.